1. Field of the Invention
This invention relates to a process for the production of alumina carrier having a remarkably large surface area and a controlled pore volume, in particular, .gamma.-alumina, through rapid crystal growth of boehmite.
Alumina is widely used as a catalyst carrier because it has high mechanical strength and a large specific surface area, which is greater than that of other substances. The activity of a catalyst is generally in proportion to the surface area thereof. Therefore, many attempts are made in this field to attain an increase of the specific surface area of alumina and to improve its catalytic activity.
.gamma.-Alumina has superior (high) thermal stability and mechanical strength and, in addition, a large specific surface area. It is well known in the art that .gamma.-alumina can be produced by calcining boehmite gel and that boehmite gel is a hydrated gel of fibrous boehmite crystallite which is referred to as "pseudo-boehmite". The boehmite gel can generally be produced by aging non-crystalline aluminum hydroxide at a temperature of 50.degree. C. or higher and a pH value of 6 to 10. If it is desirable to produce .gamma.-alumina having a large specific surface area, the crystal size of said pseudo-boehmite must be adjusted to a suitable size. Naturally, when said pseudo-boehmite has an excessively large crystal size, the .gamma.-alumina formed by calcination of said pseudo-boehmite will have a relatively small specific surface area. In addition, when said pseudo-boehmite has an excessively small crystal size, the resultant .gamma.-alumina will also have a reduced specific surface area. This is because the pseudo-boehmite crystallites are sintered during calcination. Furthermore, if a boehmite gel comprising crystals having a range of different sizes is used, there will be reduction of the specific surface area of the resultant .gamma.-alumina, because during calcination, thermally unstable boehmite crystallites sinter and, as a result of sintering, they form relatively large crystals. Accordingly, in order to obtain alumina having a very large specific surface area, it is necessary to first produce a boehmite gel which is grown to a minimum size, the size being such as not to cause sintering of the gel during calcination, and which has a uniform particle size.
In many catalytic reactions, in addition to the specific surface area having an effect on the catalytic activity, the pore size of the catalyst also has an important effect on the catalytic activity. The smaller the average pore size, the lower the diffusion rate of the reaction molecules into the pores, resulting in a decrease of the catalytic effectiveness factor and lower catalytic activity. Furthermore, when the average pore size is increased, the catalytic effectiveness factor is also increased, but an increase in the effective coefficient stops after the average pore size is increased to a specific value. If the average pore size is increased beyond said specific value, the apparent catalytic activity will be decreased as a result of a decrease in the packing density because the pore volume of the catalyst is increased. Generally, in order to provide a catalyst exhibiting maximum catalytic activity, it is necessary to produce a carrier which has both a maximum specific surface area and an optimum pore size.
2. Description of the Prior Art
The conventional methods for controlling the pore size of alumina comprise controlling the size of pores formed between the ultimate particles of alumina as a result of the control of the size of said ultimate particles. These conventional methods have a defect, namely, a reduced specific surface area of the product alumina, because alumina having an increased particle size must be provided to make a large pore size. Various methods are proposed to control shrinkage of the gel structure during drying and calcining of the boehmite gel. These methods are proposed as methods which can maintain the specific surface area of .gamma.-alumina at a high value and also control its average pore size. With regard to these well-known methods, control of the average pore size is deemed to be identical with that of the pore volume because, the specific surface area of alumina cannot be changed by these methods. Some examples of these well-known methods include (1) the method of varying the drying speed of the boehmite gel ("J. Polymer Science", Vol. 34, 129); (2) the method of applying of shearing stress on the concentrated boehmite gel (Japanese Patent Application Laying-open Gazette Sho 49-31597) and the like. These methods are disadvantageous in that the range of pore volume controllable by these methods is very narrow. Furthermore, methods for controlling the pore volume of .gamma.-alumina to widen its range are proposed and they include the (1) method of adding a water-soluble polymer such as polyethylene glycol and the like to the boehmite gel (Japanese Patent Application Laying-open Gazette Sho 52-104498 and 52-77891); and (2) the method of replacing a part of, or the major part, of water in the boehmite gel with alcohol(s) (Japanese Patent Application Laying-open Gazette Sho 52-123588) and the like. The former method relates to control the pore volume by an inhibition of dense aggregation of the boehmite crystallites by the surface tension of the water during the drying step, based on the amount of addition of the water-soluble polymer. This method is not preferred on economic grounds because the anti-aggregation agent used eventually must be calcined and removed. In addition, it is difficult using this method to prevent a decrease in the surface area due to the exothermic heat during the calcination. The latter method relates to control the pore volume of alumina by modifying the surface tension of water and, therefore, controlling the degree of aggregation of the boehmite crystallites, by varying the amount of alcohol used in place of water. The defects of this method are that apparatus for recovering the alcohol is necessary, and that the resulting alumina has poor water-resistant characteristics, and is easily cracked upon water absorption.
Methods for increasing the pore volume without using additives as described hereinbefore are also well known. These well-known methods include the method disclosed in Japanese Patent Publication Sho 49-37517, in which a part of the boehmite gel is first changed to xerogel and the xerogel is then incorporated into a hydrogel of boehmite to increase the pore volume. This method is limited to the production of alumina having a so-called "double peak"-type pore distribution, namely, alumina having small pores formed between the boehmite crystallites and large pores, for example more than 500 A, formed between the xerogels. Such catalysts having a "double-peak"-type pore distribution when compared with conventional catalysts having a single peak-type pore distribution, usually are disadvantageous in that the catalytic activity is lower.